I just have a question about wiring since I am replacing some of the factory switches in my car with 25A toggle switches. The switches are for things that have a fuse less than 25A, and control things such as the windshield wipers, door locks, and radiator fans. Can I use thicker gauge wire than stock to wire the new switches in without any problems? And do I use the same fuse?

Most importantly, how can do I know how much current each device draws? Do I just look at the size of the fuse for that circuit and determine it that way? Thanks

Before you take anything apart look carefully at the application. For example door lock switches are different from the drivers side to the passenger side, the drivers side being the 'master'. Wiper motors on most vehicles switch on/off with the ground and not the positive. Also they are two position, high/low speed.
What you want to do is feasible, just watch the application..

well, what are the main differences when you switch the ground and not the positive? What are the reasons for doing this?

Thanks!

Also, I wouldn't try this, but I just wanted to know what would happen if you have a circuit with a high current, and you used very thick wire (the CORRECT wire) for the positive, but on the negative you used extremely thin wire (24g). What would happen?

well, what are the main differences when you switch the ground and not the positive? What are the reasons for doing this?

On the wipers, for example,the ground is switched so that when they are turned off they go to the 'park' position,that's the lowest position where they're out of sight. The park switch is built into the wiper motor asssembly, it maintains the ground connection until the wipers are out of sight. Without it the wipers would stop as soon as the switch was turned off, usually in the middle of the windshield.The dome lights also use a ground switching scheme, when either door is opened the dome light gets grounded. The headlight switch,usually when turned, puts a ground on the dome lights(but not the headlights). Like I said you have to be careful..